The present invention relates to a process for the manufacture of polyamide.
It relates more particularly to the manufacture of polyamides resulting from the reaction between a diacid and a diamine.
Polyamides are polymers of great industrial and commercial importance. Thermoplastic polyamides are obtained either by reaction between two different monomers or by polycondensation of a single monomer. The invention applies to polyamides resulting from two different monomers, the most important polyamide of which is poly(hexamethylene adipamide). Of course, these polyamides can be obtained from a mixture of diacids and diamines. Thus, in the case of poly(hexamethylene adipamide), the main monomers are hexamethylenediamine and adipic acid. However, these monomers can comprise up to 25 mol % of other diamine or diacid monomers or even of amino acid or lactam monomers.
This class of polyamides resulting from two different monomers is generally manufactured by using, as starting material, an amino acid salt obtained by mixing a diacid with a diamine in a stoichiometric amount and in a solvent such as water.
Thus, in the manufacture of poly(hexamethylene adipamide), adipic acid is mixed with hexamethylenediamine in water to produce a hexamethylenediamine adipate better known under the name of Nylon salt or xe2x80x9cN Saltxe2x80x9d.
The solution of N salt is optionally concentrated by evaporating the water.
The polyamide is obtained by heating such a solution of Nylon salt at high temperature and high pressure to evaporate the water, while avoiding any formation of solid phase in order to prevent the mixture from setting solid.
This operation consumes a great deal of energy and also does not make it possible to completely control the stoichiometry as amine can be evaporated or entrained by the water. Such an entrainment or evaporation requires supervision of the process in order to reestablish the stoichiometry and can be troublesome as the diamine entrained can contaminate the effluents discharged by the production plant.
In addition, the need to heat to a high temperature under a high pressure can lead to the formation of decomposition compounds which contaminate the manufactured polyamide, in particular with regard to its color.
To avoid the use of water, processes without water have been provided without real success. This is because it is very difficult to control the stoichiometry between the two reactants when at least one of them is in a solid form.
In addition, it is impossible to feed the reactants in a liquid form as, at the temperature required, decomposition of the monomers takes place.
However, certain processes for the preparation of a polyamide without water and without solvent have already been provided. Thus, U.S. Pat. No. 4,131,712 discloses a process which consists in mixing, in the liquid state, a diacid-rich diamine/diacid mixture with a second diamine-rich diamine/diacid mixture, so as to obtain a composition which is as close as possible to stoichiometry. The mixture is maintained at a temperature sufficient to prevent any solidification and to carry out the polycondensation. However, in this process, it is difficult to control and to obtain a stoichiometric mixture as the diamine partially evaporates during the introduction of the diamine-rich mixture.
To avoid this disadvantage, a process for the manufacture of polyamide without water or solvent is disclosed in patent application WO 96/16107.
This process consists in feeding, into a first stage of a multi-stage reactor or plate column, a diacid or a diacid-rich diacid/diamine mixture and then feeding, at an intermediate stage, diamine or a diamine-rich diacid/diamine mixture while controlling the amount of diamine added in order to obtain a stoichiometric mixture, the stoichiometry being measured continuously by an analysis means, for example analysis by Near InfraRed (NIR).
This process requires the use of a special reactor for its implementation, the operation of which may prove to be difficult to control. Furthermore, the control of the stoichiometry is provided by the addition of diamine or of a diamine-rich diacid/diamine mixture, which can result in large variations in the diacid/diamine ratio and in variations in the total flow rate into the reactor.
One of the aims of the present invention is to provide a process for the manufacture of high molecular weight polyamide without water or solvent which makes it possible to control the stoichiometry, it being possible for this process to be carried out in reactors of simple design. In addition, the process is easy to direct.
To this end, the invention provides a process for the manufacture of a polyamide resulting from the reaction of at least one diacid with at least one diamine comprising the following stages:
preparing a first mixture of diacid and of diamine with a diacid/diamine molar ratio of between 0.8 and 0.995, preferably of between 0.95 and 0.99,
preparing a second mixture of diacid and diamine with a diacid/diamine ratio of between 1.005 and 1.2, preferably between 1.01 and 1.05,
introducing, in the molten form, a first stream of the first mixture and a second stream of the second mixture into a stirred polymerization reactor,
withdrawing, preferably continuously, a stream of polyamide prepolymer from said reactor,
feeding said polyamide stream into a finishing stage in order to obtain the desired degree of polymerization.
The first and second mixtures are preferably anhydrous. The term xe2x80x9canhydrous mixturexe2x80x9d should be understood as meaning mixtures which can comprise up to 10% by weight of water. The term xe2x80x9canhydrousxe2x80x9d is used in the present description in contrast with the conventional process, which uses an aqueous solution of Nylon salt.
The process of the invention also comprises a means for the continuous analysis of the mixture of the streams of the first and second mixtures, preferably the two streams, in order to determine the balance between acid and amine functional groups. The term xe2x80x9cacid and amine functional groupsxe2x80x9d is understood to mean the total acid and amine functional groups present, which have or have not reacted. This process consists in controlling, from the result of the preceding analysis, at least one of the first and second streams of mixtures, preferably both streams, in order to obtain a ratio of the acid functional groups to the amine functional groups equal to the value desired according to the type of polyamide manufactured (by way of indication, the acceptable margin of variation for this ratio with respect to the desired value is plus or minus 0.0005).
According to a preferred embodiment of the invention, the analytical means is a Near InfraRed (NIR) spectrometric analysis device.
The process of the invention thus makes it possible to obtain a balanced polyamide by controlling the stoichiometry of the mixture. Furthermore, as the adjustment or the control of this stoichiometry is carried out by the control of the streams of the first and second mixtures, the size of the variation in the acid/amine ratio will be low after the mixing of the two streams, making it possible to obtain more homogeneous production.
In addition, as indicated above, the polymerization reactor is a simple reactor comprising only conventional stirring and heating means.
The reactors in which the first and second mixtures are prepared, and the polymerization reactor, are can, for example, be reactors with mechanical stirring or reactors with external recirculation. In the latter case, the feeding with mixtures and/or the withdrawing of the product and/or heating can advantageously be carried out in the recirculation loop. The reactors can be heated using a jacket device and optionally an internal coil. In addition, the reactors can be open to gases.
According to a preferred characteristic of the invention, the first and second mixtures of diacid and diamine are prepared by mixing, in the solid state, a diacid with the diamine in the presence of a small amount of water and then heating at moderate temperature in order to obtain an amine salt.
In another embodiment, the mixtures are heated at a higher temperature with the removal of water in order to obtain prepolymers with an acid ending for the first mixture and an amine ending for the second mixture.
The process for the preparation of these mixtures may be analogous to that disclosed in U.S. Pat. No. 4,131,712 for the preparation of acid-rich mixtures.
In another preferred characteristic of the invention, the finishing stage comprises a rapid evaporation of the condensation water present in the polyamide exiting from the polymerization reactor, which evaporation is obtained, for example, by reducing the pressure of the polyamide stream. The polyamide is subsequently maintained for a predetermined time at a polymerization temperature at atmospheric pressure or under reduced pressure in order to obtain the desired degree of polymerization.
These final finishing stages are those used in industrial processes for the manufacture of polyamide from an aqueous solution of amine salt.
According to a novel characteristic of the invention, the polymerization carried out in the polymerization reactor is carried out under an autogenous or regulated pressure in order to avoid any loss of diamine or at least to reduce them to a minimum.
The process of the invention can be used for the manufacture of poly(hexamethylene adipamide) from adipic acid, as diacid monomer, and from hexamethylenediamine, as diamine monomer.
The process of the invention also makes it possible to manufacture other polyamides from a diacid monomer chosen from the group consisting of glutaric, suberic, sebacic, dodecanedioic, isophthalic, terephthalic, azelaic, pimelic and naphthalenedicarboxylic acids, for example.
Mention may also be made, as diamine monomers, in addition to hexamethylenediamine, of heptamethylenediamine, tetramethylenediamine, octamethylenediamine, nonamethylenediamine, decamethylenediamine, 2-methylpentamethylenediamine, undecamethylenediamine, dodecamethylenediamine, xylylenediamine or isophoronediamine.
The invention preferably applies to the manufacture of polyamide from diacid and diamine monomers which are respectively at least 80 mol % adipic acid and hexamethylenediamine.
It is also possible to prepare polyamides from diacid and diamine monomers comprising a low proportion (less than 20 mol %) of amino acid or lactam. Mention may thus be made of the manufacture of copolyamide PA 6,6/6 from adipic acid/hexamethylenediamine monomers comprising a predetermined amount of caprolactam. This different monomer can be present in the first or second mixtures or in only one of these mixtures.
In the embodiment of the process for the manufacture of poly(hexamethylene adipamide), the first and second mixtures are advantageously prepared from an adipic acid (AdOH)/hexamethylenediamine (HMD) mixture with a composition close to that of the eutectic, that is to say 19% by weight of HMD and 81% by weight of AdOH.
This eutectic mixture has a melting point in the region of 100xc2x0 C.
The eutectic mixture is introduced into two reactors into which hexamethylenediamine is introduced in order to obtain respectively a first mixture rich in diacid (diacid/diamine ratio of between 1.005 and 1.2) and a second mixture rich in diamine (diacid/diamine ratio of between 0.8 and 0.995).
According to another embodiment of the invention, the first and/or second mixtures are themselves obtained by mixing two premixers of diacid and of diamine, the proportions of which are within ranges from 0.5% to 20% respectively above and below the value targeted for each mixture. For each mixture, the streams of the two premixes are automatically controlled according to the value of the ratio desired for said mixture. According to this embodiment, the accuracy of the stoichiometric ratios of the mixtures and their control are improved. According to this embodiment, the device for the implementation of the invention comprises a cascade of several stages of reactors in which the accuracy of the stoichiometry is increasingly important. The number of stages of reactors can be between 1 and 10. The premixes can themselves be prepared analogously to the preparation of the mixtures, for example from an AdOH/HMD mixture for the composition close to that of the eutectic.
Advantageously, the diacid/diamine ratios are controlled by chemical or potentiometric analyses. In a particularly preferred embodiment, these diacid/diamine ratios are determined by a Near InfraRed spectrometric analysis.
The temperature for the preparation of the mixtures is advantageously greater than 200xc2x0 C. in order to obtain an amidation reaction and thus a prepolymer with an acid or amine ending, according to the mixture.
According to one embodiment of the invention, the first and second mixtures thus obtained are withdrawn from the reactors as separate streams A and B, and these two streams are combined and introduced into a stirred polymerization reactor.
It is possible to provide static mixers in the feed pipe of the polymerization reactor and/or a premixing reactor.
The flow rate of each stream A and B is specified in order to obtain a ratio of the amine functional groups to the acid functional groups which is as close as possible to the desired value.
According to the invention, the flow rate of the streams is controlled and adjusted by automatic control according to the result of the analysis of the ratio of the acid functional groups to the amine functional groups present either in the reaction mass formed by the mixing of the two streams, for example in the premixing reactor or after the static mixers, or in the reaction mass present in the polymerization reactor or at the outlet of said polymerization reactor.
The regulation of the flow rates for entry into the various reactors can be carried out via pumps or pressure-reducing valves.
To obtain efficient regulation of the stoichiometric ratio, this analysis of the acid and amine functional groups is carried out continuously by Near InfraRed spectrometric analysis. The automatic control can also be related to the measurement of stoichiometric ratio in the two streams M1 and M2.
This analytical technique is disclosed in particular in U.S. Pat. No. 5,155,184 for the determination of a property of a polymer, such as a polyolefin. It consists briefly in measuring the absorbance of a light beam by a sample in a wavelength range between 800 and 2 600 nm and in producing a calibration curve, identification being carried out by chemometry of the differences in absorbance at different wavelengths and the variations in a property P measured by a chemical or physicochemical analytical method.
This technique for determining a property of a polymer has also been disclosed in U.S. Pat. No. 5,532,487 and WO 96/16107. Thus, in U.S. Pat. No. 5,532,487, the Near InfraRed spectrometric analysis method is used for the determination of the concentrations of the acid and amine end functional groups in a polyamide in the solid state, for example on a yarn, or on anhydrous diacid/diamine mixtures.
Likewise, patent WO 96/16107 discloses the use of a Near InfraRed spectrometric analysis method for determining the concentration of acid and/or amine end functional groups in a polyamide in a molten medium at the reactor outlet. However, in both these examples, the polyamide analyzed is substantially anhydrous.
In the process of the invention, this determination of the concentration of acid and/or amine end functional groups is carried out by analysis of the reaction mass comprising the water resulting from the amidation reaction, for example in a bypass loop of the main stream, in an optional bypass of the main stream, or in a recirculating loop of a reactor.
The process of the invention thus makes possible the manufacture of a polyamide from diamine and diacid without use of water or solvent in reactors of simple design and which are easy to control.
Furthermore, as the control of the stoichiometry is obtained by varying the feeding of mixture comprising either a slight excess of acid or a slight excess of amine, the variations in the acid/amine ratio will be small. Thus, the operation of the plant is more regular, without large-scale variation.